what other pressure do you have the ability to put on a billionaire that they wouldn't find utterly laughable, if they even noticed it? but you can't buy your way out of death. sure, if the threat to their lives were to become credible, they could leverage their wealth to protect against it, but being surrounded by bodyguards at all times, having every rooftop surveyed for snipers before you go out to get coffee - these are things that disrupt their overly-cruisy existences. and the more people there are gunning for them, figuratively or literally, the worse their lives get.

and sure, if you off them, their wealth will just default to someone else. but if billionaires start dying left and right, their inheritants might just start to find ways to make sure their fortune stays a little shy of that magical 9-digit mark.